1. Field of the Invention
The field of the present invention relates to medical testing to determine the condition of an implanted heart valve in order to determine whether the implanted heart valve is intact and is not in imminent danger of malfunctioning due to structural defects or whether the implanted heart valve is in various stages of possible failure which necessitates the replacement of the valve through surgery.
2. Description of the Prior Art and Discovered Defects in the Bjork Shiley Convexo-Concave Mechanical Heart Valves
2.1 General Prior Art Background
The following prior art references are listed in superscript in the text below:    1. Bjork V O, “A new tilting disc valve prosthesis,” Scand J Thorac Cardiovasc. Surg. 3, 1-10 (1969).    2. Bjork V O, “The improved Bjork-Shiley fitting disc valve prosthesis.” Scand. J. Thorac. Cardiovasc. Surg. 12. 81-84 (1978).    3. Bjork V O, “The optimal opening angle of the Bjork-Shiley tilting disc valve prosthesis,” Scand. J. Thorac. Cardiovasc. Surg. 15, 223-227 (1981).    4. Wieting D W, Eberhardt A C, Reul H, Breznock E M, Schreck S G. Chandler J G. “Strut fracture mechanisms of the Bjork-Shiley convexo-concave heart valve,” J. Heart Valve Dis. 8, 206-217 (1999).    5. Wenzel T C, Mannig C R, Chandler J G, Williams D F, “Welding metallurgy's putative influence on Bjork-Shiley convexo-concave valve outlet strut failures.” J. Heart Valve Dis. 8, 218-231 (1999).    6. Walker A M, Punch D P, Sulsky S I, Dreyer N A. “Manufacturing characteristics associated with strut fracture in Bjork-Shiley 60 degrees convexo-concave heart valves,” J. Heart Valve Dis. 4, 640-648 (1995).    7. Kallewaard M, Algra A, Defauw J. van der Graaf Y, “Which manufacturing characteristics are predictors of outlet strut fracture in large sixty-degree Bjork-Shiley convexo-concave mitral valves?” The Bjork Study Group. J. Thorac. Cardiovasc. Surg. 117. 766-775 (1999).    8. Blot W J, Omar R Z, Kallewaard M, et al. “Risks of fracture of Bjork-Shiley 60 degree convexo-concave prosthetic heart valves: Long-term cohort follow up in the UK, Netherlands and USA,” J. Heart Valve Dis. 10, 202-209 (2001).    9. de Mol B A, Overkamp P J, van Gaalen G L, Becker A E, “Non-destructive assessment of 62 Dutch Bjork-Shiley convexo-concave heart valves,” Eur. J. Cardiothorac. Surg. VI, 703-708. discussion 708-709 (1997).    10. Candy J V, Jones H E, “Processing of prosthetic heart valve sounds for single leg separation classification,” J. Acoust-Soc. Am. 97, 3663-3673 (1995).    11. Plemons T D, Hovenga M, “Acoustic classification of the state of artificial heart valves,” J. Acoust. Soc. Am. 97, 2326-2333 (1995).    12. Eberhardt A C, Chassaing C E, Ward M A, Lewandowski S J, “Acoustic characterization of mechanical valve condition and loading,” J. Heart Valve Dis. 4, 649-658, discussion 658-659 (1995).    13. Dow J J, Plemons T D, Scarbrough K, et al. “Acoustic assessment of the physical integrity of Bjork-Shiley convexo-concave heart valves,” Circulation, 95, 905-909 (1997).    14. O'Neill W W, Chandler J G, Gordon R E, et al. “Radiographic detection of single strut leg separations in Bjork-Shiley convexo-concave mitral valves.” N. Engl. J. Med. 333, 414-419 (1995).    15. Chandler J G, Hirsch J L, O'Neill W W, et al. “Radiographic detection of single strut leg separations as a putative basis for prophylactic explanation of Bjork-Shiley convexo-concave heart valves,” World J. Surg. 20, 953-959, discussion 959-960 (1996).    16. Hopper K D. Gilchrist 1C, Landis J R, et al. “In vivo accuracy of two radiographic systems in the detection of Bjork-Shiley convexo-concave heart valve outlet strut single teg separations.” J. Thorac. Cardiovasc. Surg. 115, 582-590 (1998).    17. de Mol B A, Cromkeecke M E, Groen J G, Faber G, van der Heiden M S, Ongkiehong L, “The complexity of external acoustic detection of defects in Bjork-Shiley convexo-concave heart valves,” Artif. Organs 25, 63-67 (2001).    18. Fatemi M, Greenleaf J F, “Vibro-acoustography: An imaging modality based on ultrasound-stimulated acoustic emission,” Proc. Natl. Acad. Sci. USA 96, 6603-6608 (1999).    19. Flannery B, Deckman H, Roterge W, D'Amico K, “Three-dimensional x-ray microtomography,” Science 237, 1439-1444 (1987).    20. F. Burny, “Monitoring of Orthopedic Implants: A Biomaterials-Microelectronics Challenge,” Elsevier Science, 1993.    21. M. A. Barbosa, “Imaging Technics in Biomaterials: Digital Image Processing Applied to Orthopedic and Dental Implants,” Elsevier Science, 1994.    22. P. M. Morse and U. Ingard, “Theoretical Acoustics,” McGraw-Hill, New York 1968.    23. L. F. Kinsler et al., “Fundamentals of Acoustics” (4th ed.) John Wiley, NY 1999.    24. G. R. Tor, “The Acoustic Radiation Force” Am. J. Phys. 52, 402-408 (1984).    25. R. T. Beyer, “Radiation Pressure in a Sound Wave” Am. J. Phys. 18, 25-29 (1950).    26. (a) J. D. N Cheeke, “Fundamentals and Applications of Ultrasonic Waves,” CRC Print 2002; (b) P. M. T. Wells, “Biomedical Ultrasonics,” Academic Press, New York, 1977.    27. J. F. Greenleaf and M. Fatemi-Booshehri, “Acoustic Force Generation for Detection, Imaging and Information Transmission Using the Beat Signal of Multiple Intersecting Sonic Beams,” (May 11, 1999) U.S. Pat. No. 5,903,516.    28. J. F. Greenleaf and M. Fatemi-Booshehri, “Acoustic Force Generation by Amplitude Modulation of Sonic Beams,” (Jul. 13, 1999) U.S. Pat. No. 5,921,928.    29. M. Fatemi and J. F. Greenleaf, “Vibro-Acoustography: An Imaging Modality Based on Ultrasound-Stimulated Acoustic Emission,” Proc. Natl. Acad. Sci. 96, 6603-6608 (1999).    30. Chia, R., “Finite Element Analysis of Vibrations of the Bjork Shiley Convexo-Concave Heart Valve,” Seventh Annual IEEE Symposium on Computer-Based Medical Systems, 1994, 48-52    31. Avrom Brendzel, Edmond Rambod, Steven M. Jorgensen, Denise A. Reyes, Michael Chmelik, Erik Ritman, “Three-dimensional Imaging of Fractures in Outlet Struts of Bjork Shiley Convexo-Concave Heart Valves by Micro-computed Tomography in vitro”, J. Heart Valve Dis, Vol 11. No. 1, January 2002, 114-120.
The Bjork-Shiley Convexo-Concave (BSCC) heart valve was initially approved for patient use in 1979. The BSCC valve was suggested to be an improvement on the earlier RS (radial spherical) model based on a more physiologic flow profile and reduced thrombogenicity.1-3 Soon after its introduction isolated fractures of the outlet strut began to be reported and their continued occurrence led to the withdrawal of the valve in 1986. During that seven-year period over 86,000 valves were implanted. This followed a number of reports of valve dysfunction due to structural failure, leading to patient death. Failure risk assessments have been reported.6-8 To date more than 600 fractures have been reported with roughly 60% being fatal.
Fatigue and modeling studies have indicated that outlet strut fracture occurs in two phases with initial fracture of one of the legs of the outlet strut (Single leg separation or SLS) followed after a variable period of time by fracture of the second leg with resultant separation of the outlet strut, escape of the occluding disk and valve failure. Different technologies have been used to device a diagnostic that will identify structural imperfections in the BSCC heart valve.4 
The current technology focuses on the active acoustic method in which an external source generates sound waves at varying, controlled frequencies within the range of natural resonance of the outlet struts. Finite element modeling, experimental, and in vitro flow studies have shown that the outlet strut vibrates at different characteristic frequencies when it is intact (˜7 KHz), or when it is cracked (2 KHz and ˜4 KHz)11. Insonifying the strut at a close or similar frequency by an external source will induce resonance that can be detected by an external sensor and thereby define the physical state of the outlet strut. The active acoustic approach has the theoretic advantage that the outlet strut can be excited and the resulting vibrations (emitted frequencies) can be sampled during a specific period within a cardiac cycle when the valve is closed. The resulting signals are thus free of noisy valve opening and closure sounds. This controlled classification method is in contrast to passive acoustic methods (the detection of the random vibrations induced in the outlet strut by opening and closure of the disc) where vibrations are only present during a limited time window and require sophisticated signal extraction and noise-elimination procedures to separate the desired frequencies from the higher amplitude disc impact signals.
2.2 Description of the BSCC and Prior Art Test to Attempt to Locate Structural Defects in the Implanted BSCC
In order to describe the invention in full, we will provide a detailed description of one kind of implanted heart valve substitutes namely, the Bjork-Shiley Convexo-Concave (BSCC) mechanical valve. Periodic evaluation of the health of this valve is very critical and present imaging diagnostics modalities have failed to reliably verify the status of the valve integrity, by discovering the possible existence of microscopic fractures of the outlet strut. The BSCC valve (FIG. 1) consists of the following components, a flange (orifice ring) 1 and an inlet strut 4 fabricated as a single piece from a bar of Haynes 25 alloy; an outlet strut 2 formed from Haynes 25 alloy wire; a pyrolytic carbon-coated graphite disc 3 that occludes blood flow in its closed position, and allows blood flow in its open position; and a fabric sewing cuff 5 that surrounds the flange and that is sutured to the patient's heart tissue at implant. The outlet strut wire was formed into a W-shape, with the free ends of the legs of the W being attached to the flange by tungsten-inert-gas welding without filler material.5, 9 The outlet strut and inlet strut limit the motion of the disc and prevent its escape as it passively cycles between the open and closed positions. The disc opens to either 60° or 70°, depending on the specific BSCC model.7 Embedded within the disc is a tantalum C-ring radio-opaque marker. The mitral and aortic implant versions of the BSCC valve differed only in the shape of the sewing cuff. The valve was supplied in several sizes, with sizes 31 and 33 mm using the same metal and disc components (‘valve body’) as the size 29 mm valve, but with larger sewing cuffs.
Valve dysfunction consists of embolization of the disc due to structural failure of the outlet strut (outlet strut fracture, OSF), which has been attributed to a two-step fatigue process induced by cardiac cycle loading. In the first step, a fatigue crack develops in one leg of the outlet strut, and after a currently unpredictable length of time (which varies from case to case), leads to a through-fracture in that leg—a condition called single-leg separation (SLS). After SLS develops, the continued structural integrity of the other leg prevents disc escape. Wear burnishing of the fracture faces in the SLS leg occurs during the period that the other leg maintains its integrity.5 In the second step, which may not occur in each BSCC valve having an SLS strut (‘SLS-BSCC valve’), a fatigue crack develops in the second leg and, after an additional unpredictable and variable length of time, that leg also experiences fatigue failure, leading to disc escape. Because only a small fraction of BSCC valves apparently will fail by OSF during the patient's natural lifetime, and since valve replacement surgery itself carries a significant risk, it is desirable to develop a rationale for prophylactic BSCC valve replacement. To identify patients with defective valves, several groups have investigated screening methods including passive acoustic monitoring of valve closing or opening10-13 and radiographic imaging of fractured struts.14-16 However, so far such methods have not been able to reliably detect SLS-BSCC valves with sufficient specificity in vivo.16-17 
Variations in outlet strut fracture morphology, including partial-thickness fractures (cracks), fracture faces in contact, fracture faces separated, and laterally displaced fracture faces, and variations in the position of fracture relative to the welded section of the strut, have been reported for a group of SLS-BSCC valve explants that were examined by optical and scanning electron microscopy.9 However, the fracture faces of such SLS outlet struts cannot be imaged without sectioning the struts, which would prevent further evaluation of the spatial relations between the strut and other valve components. Imaging of the fracture faces of a group of OSF outlet struts by optical and scanning electron microscopy to conduct fractographic examinations has been reported.5 The fractographic observations include striations indicating fatigue failure, and fracture initiation sites were identified on inflow sides only, commonly in the weld but sometimes in the heat-affected (near-weld) zone or the base metal. Other methods like X-ray micro-computed tomography (micro-CT) were used to obtain accurate, high-resolution three-dimensional (3-D) images of small objects.31 The micro-CT imaging modality was used to characterize non-destructively the morphology and position of fractures in the outlet struts of SLS-BSCC valve in vitro.31 This approach was pursued because CT is superior to regular projection X-ray imaging for several reasons.31 One is that projection along one axis may be unable to detect a generally planar crack or fracture when fracture faces are nearly in contact, since the radio density along any one projection axis may be nearly constant. Moreover, even if a fracture is detected with projection X-ray imaging, no information on 3-D features of the fracture faces, such as fatigue striations, or wear burnishing, can be obtained. Furthermore, an accurate measurement of the distances separating the fracture faces can best be obtained from a high-resolution 3-D image as provided by micro-CT. Although, the application of micro-CT to image outlet strut fractures in BSCC valves explants demonstrates the value of this method for fracture characterization in vitro, including visualization of the fracture faces of the SLS struts without physical sectioning. Consequently, this method is not suitable for clinical use, because it requires high-intensity, dangerous X-ray radiation. Micro-CT can serve as a tool to understand the failure mechanisms, and perhaps to aid in the development of clinical differentiation methods.31 